Science Puzzle
Why Do Geysers Erupt on a Timetable?
Old Faithful in Yellowstone National Park blasts a column of boiling water and steam up to about 55 metres into the air for a few minutes, then goes quiet. Some time later, usually after about an hour and a half, it does it again, so regularly that park rangers post the time of the next eruption for visitors.
What makes a geyser erupt, and why do the eruptions come round so regularly?
The Answer
Deep underground, water is heated well past its normal boiling point but can't boil because of the weight of water above it. When it finally starts to boil, it sets off a chain reaction that empties the pipe in one great blast. Refilling and reheating take roughly the same time each cycle, so the eruptions keep a rough timetable.
Rainwater and snowmelt seep down through cracks into a system of narrow pipes and chambers sitting above very hot rock. Yellowstone sits on top of a vast body of hot and partly molten rock. Water near the surface would boil at about 93 °C (Yellowstone is high up, where the air pressure is lower), but deep down the weight of the water column squashes it, raising its boiling point, so it can heat to well over 100 °C without boiling. Water like this is called superheated.
Eventually some of it does begin to boil. The steam bubbles rise and push water out of the top of the narrow pipe. That lowers the weight pressing on the superheated water below, so its boiling point drops below its actual temperature, and in an instant a huge amount of it flashes into steam. The expanding steam shoves everything above it out of the vent: an eruption.
Then the system has to refill and heat up again, which takes a similar time every cycle. Old Faithful's gaps range from about 35 to 120 minutes, averaging about 90. Rangers predict the next one from how long the last eruption lasted: a long eruption empties more water and is followed by a longer wait.
The principle: Superheating and sudden pressure release. Pressure raises water's boiling point, so deep water can be far hotter than 100 °C without boiling, and when the pressure is suddenly reduced it flashes into steam all at once.